The whole experimental process, including sample preparation, transferring, and characterization were performed in an ultra high vacuum (UHV) system with a base pressure better than 3 × 10−10 torr. The Si(111) 7x7 template was prepared by repeated flashing at 12000C [3,40,41]. For LT and RT-growth, the Fe atoms were deposited while the substrate was at 100 K and 300 K, respectively. The film thickness was calibrated by using Auger electron spectroscopy (AES) and scanning tunneling microscope (STM). Both the detections of AES and STM were performed while the sample was at room tem-perature. The magnetic behavior was investigated by using magneto optical Kerr effect (MOKE) in both longitudinal and polar directions with the lock-in technique. The MOKE measurements shown in this report were performed at RT.
Figure 4.1: The whole experiment process
The whole process is shown in Fig. 4.1. Because of the different resistance of all Si slabs we need to adjust the current pass through Si carefully to avoid the sample overheated. We can know the tempera-ture of sample by the thermal couple or the power we add on it. We have to deal properly with the Si slab by heating at 6000C for more than 6 hours to completely get rid of the oxidation. Then we measure the AES to check if the sample is clean enough.
Figure 4.2: 12 ML RT-growth Fe/Si(111). The signal of carbon is much smaller than iron and silicon after 12 ML Fe deposition.
Figure 4.3: The process of preparation Si(111)7x7
The last process is the key to obtain large Si(111)7x7 terrace. We
need to reduce the current very slowly, or said, cooling down slowly, to get the large terrace. By STM image, we can confirm if the surface is Si(111) 7x7 and the terrace is large enough. In Fig. 4.4, we can see a terrace with the width about 30nm and its reconstruction of Si(111)7x7.
Figure 4.4: Si(111) 7x7 image obtained in our laboratory
Up to now, we have obtain the substrate for following experiment.
We deposit the Fe on Si(111) at RT and measure its AES data as the control group. In our system, we use LT-grown films as the buffer layer and then deposit Fe at RT several times. We measure AES, MOKE and STM data after Fe deposited on Si substrate every time.
4.1 LT(100K)-growth of Fe/Si(111)
In order to investigate the different interface condition of LT and RT-grown Fe on Si(111), the coverage-dependent AES detection was performed. Fig. 4.5 shows the AES signal ratio of F e47eV/Si92eV (left axis) as a function of Fe thickness for RT-Fe films on Si(111) and on 5 ML LT-Fe/Si(111). The AES measurements were carried out at RT.
This means for LT-grown films, the sample underwent RT annealing before AES detection. In RT-grown Fe/Si(111) films, the Fe signal becomes observable after 3-4 ML, and then reaches the comparative signal intensity of Si at ∼9 ML. After 10 ML, the Fe signal dominates Si. The question of when the Si really disappears from the surface can be answered by seeing from the right axis of Fig. 4.5, the AES ratio of Si92eV/F e47eV. The Si/Fe ratio gradually approaches 0.06 ± 0.03 at 15 ML. This is the minimum ratio of Si/Fe we could observe.
The small and broad feature of Fe at 87 eV partially overlaps with the possible small Si-92 eV peak. The overlapping actually prevents us from checking the presence of Si more precisely. Thus, we can conclude, at least to the limit of our AES measurement, the Si(111) substrate is fully covered after deposition of 15 ML RT-grown Fe.
For comparison, the solid squares in Fig. 4.5 indicate the Fe/Si AES ratio of RT-Fe/5 ML LT-Fe/Si(111). Apparently, the Fe/Si ra-tio of 5 ML LT-Fe/Si(111) is much larger than 5 ML RT-Fe/Si(111).
Sequential deposition on 5 ML LT-Fe/Si(111) gradually increases the Fe/Si ratio in the similar trend of RT-Fe films. The horizontal shift
Figure 4.5: Auger ratios of Fe47eV/Si92eV (left axis) and Si92eV/Fe47eV (right axis) measured as a function of Fe thickness. The thickness means the total thickness of Fe deposition on Si(111). The solid circles and solid squares indicate the Fe/Si AES ratio of RT-grown Fe films and RT-Fe/5 ML LT-Fe films on Si(111), respectively.
The open circles and open squares indicate the Si/Fe AES ratio of Fe and RT-Fe/5 ML LT-Fe films on Si(111), respectively. The solid and dotted curves are the fitting results by Eq. (4.1) and (4.2).
between the two series of data is 3.5 ± 0.5 ML. A recent study of Kataoka et al. [23] showed, for RT-grown Fe/Si(111), metastable FeSi with the CsCl-type structure persists at least to 6 ML of Fe. After-ward, the bcc-Fe(111) film starts to grow. Our AES data supports their conclusion of serious silicide formation in RT-growth. Based on Kataoka et al.’s observation [23], the equation of Fe/Si AES ratio is written down by assuming the first 6 ML of Fe was mixed with Si, forming 12 ML of FeSi.
Figure 4.6: Schematic illustration of AES electron emission.
Fig. 4.6 is the schematic illustration of AES electron emission.
The quantitative AES analysis was depicted by assuming that the possibility of AES electron to emit out of the sample without losing its characterizing kinetic energy decays as e−1 of the original one after penetrating the depth of a mean free path (λ). Because of the FeSi formation, the intensity of Auger signal is contributed to the Fe films and half of the FeSi films.
(F e path of Fe47eV and Si92eV Auger electrons. IIF e
Si is the relative sensitivity between Fe and Si Auger intensity. Note, Eq. (4.1) is for RT-grown
Fe/Si(111) with t ≥ 6 ML. Besides, for RT-Fe/5 ML LT-Fe/Si(111), the Fe/Si AES ratio is written down by assuming no silicide formation at the LT-Fe/Si interface.
First, the RT-Fe/Si(111) data was fitted by Eq.(4.1), using the relative sensitivity IIF e
Si=0.66±0.06, which is consistent with the value in the Auger handbook. [42] The fitting curve matches the experimen-tal RT data well, as shown in Fig. 4.5 with λF e = 3.6 ± 0.3 ML, and λSi = 4.0 ± 0.3 ML. With these parameters, the curve of Eq. (4.2) also matches the experimental data of RT-Fe/5 ML LT-Fe/Si(111). Actu-ally, within the error bars of AES data, we cannot totally exclude the possible presence of 1-2 ML Fe-silicide formation at the inter-face. However, when considering the detailed chemical composition and AES signal of the Fe-silicide interface, the suppression of silicide formation by LT-Fe is even larger than the shift of 3.5±0.5 ML in Fig. 4.5. Our experimental and fitting results all indicate the LT-Fe intermediate layer indeed effectively reduces the Fe-silicide thickness by at least 4 ML Fe (∼8 ML FeSi) compared with RT-growth. Even after RT-annealing, the LT-grown intermediate layer can sustain the Fe/Si interface and prohibit further silicide formation.
Since LT deposition of Fe on Si(111) can effectively reduce the alloy formation at the interface even after RT annealing, we were cu-rious about the thermal stability of the LT-grown films. Fig. 4.8
shows the AES ratio of Si/Fe as a function of annealing temperature for 5-25 ML LT-Fe films. All the AES detections were performed af-ter keeping the sample at the annealing temperature for 10 mins by passing through the current directly and then cooling it to RT natu-rally. Fig. refc shows the relation of current and temperature. For 5 and 15 ML LT-Fe, the AES ratio remains invariant during 300-350 K and then we observe the increased Si signal at ∼400 K. For 25 ML LT-film, the Si/Fe ratio remains stable up to a higher temperature of 400 K. For a thicker film, like 25 ML LT-Fe, it is hard to judge if the higher stability is truly because of the higher thickness, or only because the coverage is too thick to detect the slight changes at the Fe/Si interface. Nevertheless, we can still conclude 5-15 ML LT-grown Fe films annealing to RT are stable at least up to 350 K. It means that the composition does not vary with temperature before 350 K. It also supports our suggestion of using LT-grown layer as an intermediate layer for subsequent RT-growth or processing.
The previous studies of Gallego et al. and Tsushima et al. con-cluded the Fe(111) films grows epitaxially up to a thick region, main-taining the bcc structure. [1,16,23,41] Besides the crystalline structure, the study of surface morphology is still lacking, especially for LT-Fe/Si(111) films. Usually the LT-deposition of thin film is expected to result in statistically flat morphology, due to the low mobility and random landing distribution of deposited atoms. Recent studies of quantum well systems also report the 2-step growth, meaning LT-deposition with post-annealing may lead to atomically flat surface
Figure 4.7: The relation of the current passing through the sample and the tem-perature.
Figure 4.8: Auger ratios of Si/Fe mea-sured as a function of annealing temper-ature. All the LT-grown Fe films are an-nealing to RT at first and then anneal-ing to different temperature. The cir-cles, squares, and triangles indicate the series data of 5, 15 and 25 ML LT-grown Fe/Si(111) films. The solid lines are guides to the eye.
morphology in some cases of the thin film system. Thus, we try to probe the growth result of LT-deposited Fe films on the Si(111)7x7 surface. For example, the STM images of 24 ML and 40 ML LT-Fe/Si(111) are shown in Fig. 4.9 with the line profiles. For 24 ML LT-Fe, the surface was composed of elongated and winding islands.
One can see it more clearly from the inset. The width of the islands is 4 ± 1 nm and the length ranges from 5 to 15 nm. The surface corrugation observed from the line profile is only within 0.4 ± 0.2 nm. For the higher coverage of 40 ML LT-Fe/Si(111), the morphology
changes to be composed of larger islands, which are no longer elon-gated, with the diameter = 8 ± 2 nm. The surface corrugation also increases to 0.6±0.2 nm. Actually, our STM investigation of 24-40 ML 100K-deposited Fe/Si(111) reveals similar results as 2.5 and 20.4 ML 150K-deposited Fe/Au/Si(001), which were previously reported by F.
Zavaliche et al.. [21] In their study of 2.5 ML Fe, the surface was also composed of small coalesced islands of 0.4 nm height and ∼ 6−8 nm in size. Increasing the thickness to 20.4 ML Fe only enhances the rough-ness to ∼0.6 nm. In contrast to the LT-growth, previous RT-growth results of Fe/Si(111) and (001) all reveal a relatively rough surface.
For example, the morphology of 20.4 ML RT-Fe/Si(001) consists of elongated islands of 1.0-1.5 nm height. [20]
4.2 MOKE measurement
In the above sections, compared with RT-grown films, LT-growth can effectively reduce the Fe-silicide formation at the interface, and lead to a flat surface morphology. Thus, for the magnetic study, we used a 5 ML LT-grown Fe film as an intermediate layer for the sequen-tial RT-deposition, since RT-deposition is easier to perform, especially in future applications. The MOKE hysteresis loops for n ML RT-Fe/5 ML LT-Fe/Si(111) measured at RT are shown in Fig. 4.11. The hys-teresis loops are close to the square shape. Both the Kerr saturation signal (Ms) and coercivity field (Hc) increase with Fe coverage, but in different ways. The quantitative analysis is shown in Fig. 4.12. Ms
Figure 4.9: Surface morphology of (a) 24 ML and (b) 40 ML LT-grown Fe films, investigated by scanning tunneling microscopy (STM). The figures are all of 120x120 nm2. The insets (30x30nm2) reveal the detailed surface structures. The line profiles indicated in the STM images are plotted at the bottom.
increases linearly with Fe coverage. The extrapolation of the linear fitting line indicates that the magnetic dead layer could be 3.5 ± 0.5 ML in our RT-MOKE measurement. Thus, we expect, at low tem-perature, the magnetic dead layer might be even smaller than 3 ML, which can be due to a slight Fe-silicide formation or the extensive hybridization of Si electronic states into Fe.
Figure 4.10: Polar MOKE hysteresis loops measured at room temperature (RT) for n ML of RT-grown Fe on a intermediate layer of 5 ML LT-grown Fe on Si(111).
Figure 4.11: Longitudinal MOKE hystere-sis loops measured at room temperature (RT) for n ML of RT-grown Fe on a in-termediate layer of 5 ML LT-grown Fe on Si(111).
In Fig. 4.12, Hc increases monotonically and gradually becomes saturate around 70 Oe. To deduce more detailed insights from the coverage dependent evolution of Hc , we use the Stoner-Wohlfarth single domain model to simulate the Hc evolution behavior with Fe film thickness. The energy (E) correlated to magnetism in our thin film system can be written as followings. [43,44]
E = −H×M ×cos(θ − φ) + Kef f×sin2θ (4.3)
Figure 4.12: Kerr saturation signal (Ms: left-axis) and coercivity field (Hc: right axis) plotted as function of Fe coverage. The data points were analyzed from the hysteresis loops of n ML RT-Fe/5 ML LT-Fe/Si(111) in Fig. 4.11. The solid line and curve are the fitting results of Ms and Hc using a linear function and Eq.4.6, respectively.
θ and φ are the rotation angles of magnetic moment and magnetic field, respectively, relative to the surface normal direction. The first term is Zeeman energy. M is the averaged magnetic moment per unit volume and H is the applied magnetic field. In the longitudinal MOKE measurement, as shown in Fig. 4.11, we apply the magnetic field in the in-plane directions, and thus φ only can be ±π/2. Kef f in the second term is the uniaxial magnetic anisotropy. Since the easy axis is in the surface plane, one should expect Kef f < 0, which we can check later from the fitting results. By minimizing the magnetic energy in Eq.4.3, the magnetization direction θ can be determined as a function of the applied field H. Since we have φ = ±π/2, the
minima of E(θ) are always at ±π/2. This can be easily checked by solving the equation of dE(θ)/dθ=0 with φ = ±π/2. Therefore, in this model, the magnetization only switches between θ = ±π/2, resulting in the square magnetic hysteresis loops, which are actually consistent with our measured loops. Further, from Eq.4.3 we can deduce the coercivity field Hc at which the magnetization direction switches. For example, if the magnetic moment is at θ = π/2 initially, we apply the field in the φ = −π/2 direction to switch the magnetic moment. The coercivity field can be solved from the following equation.
d2E(θ, φ = −π/2)
dθ2 |θ=π/2 = 0 (4.4)
Hc = −2×Kef f/M (4.5)
In a thin film system, we have both the surface and the volume-contributed magnetic anisotropy. The magnetic anisotropy Kef f can be decomposed as Kef f=kv+ks/t, [44] where kv and ks are volume and surface anisotropy terms, respectively. Since Kef f is the anisotropy en-ergy per volume, the surface contribution ks will gradually decay with 1/t, where t is the thickness of thin film. Then we rewrite the Hc as a function of film thickness t.
Hc(t) = − 2
M×(kv + ks
t ) (4.6)
We take the magnetic moment of Fe: M = 2.5 µB/atom from the literature, [45] and then set kv and ks as the free parameters. The solid curve in Fig. 4.12 is the best fitting by using Eq.4.6 with kv = −0.5 ± 0.1µeV /atom and ks = 1.9 ± 0.1µeV /atom. The fitting curve qualitatively reproduces the monotonic increase and saturation
of Hc with the Fe film thickness. With these fitted kv and ks val-ues, Kef f=kv+ks/t is always negative after 5 ML. This is consistent with the in-plane magnetization of the measured Fe films. Besides, Kef f changes signs from positive to negative at the critical thickness of tc = −ks/kv = 4 ± 1M L, indicating the spin reorientation tran-sition (SRT) from surface normal to in-plane. The SRT and tc is actually consistent with the previous study of Nazir et al.. [3] They reported the perpendicular to in-plane SRT at 3.6-5.5 ML for 100K-grown Fe/Si(111) films.
Compared with other SRT systems, our fitted Kv and Ks are rel-atively small, but actually close to the crystalline anisotropy of body-centered-cubic (BCC) Fe. [43,44] One possible reason could be the domain wall motion, which is not considered in the single domain model. Another possibility is the anisotropy model could be inter-preted in another way. During the magnetization switching, instead of going through the surface normal direction, the magnetic moment may undergo 180◦ rotation just on the surface plane. In this case, we can redefine the θ and φ relative to another direction, which is perpen-dicular to the applied magnetic field, but on the surface plane. Then we can still keep the same calculation and fitting. The only difference is the magnetic anisotropy is no more the energy difference between surface normal and in-plane directions, but between the different di-rections on the surface plane. Even after modifying the anisotropy from uniaxial to 6-fold symmetry, [17] we will still get similar results of kv and ks. We have to agree, from our experiment data, it is hard
to judge which is the real situation in our system. Further work will be needed for a more advanced conclusion. However, we can at least conclude the volume and surface contributions, kv and ks, are of op-posite signs and compete with each other while the film thickness is increased. Such kind of situation eventually results in an Hc increase and saturation in our measured n ML RT-Fe/5 ML LT-Fe/Si(111) system.
Chapter 5 Conclusion
LT-deposition of Fe/Si(111) films was performed in this experi-ment.
1. LT-deposition can effectively reduce the Fe-silicide formation at the Fe/Si interface compared with conventional RT growth.
2. The LT-grown Fe films reveal relatively flat surface morphology and its interface condition remains stable around room tempera-ture.
3. The roughness of LT-grown Fe films is within 2-3 atomic mono-layers even for 24-40 ML LT-Fe films.
4. A single domain model of magnetic anisotropy is proposed to fit the magnetic coercivity evolution with film thickness. We deduce the values of surface and volume-contributed magnetic anisotropy, which are inverse signs and their competition is the main origin for the variation in coercivity.
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